1. Field of the Invention
The principles of the present invention generally relate to backlight units. More particularly, the principles of the present invention relate to high efficiency backlight units having external electrodes with increased lengths.
2. Discussion of the Related Art
Generally, cathode ray tubes (CRTs) are one type of flat display device that has been widely used in applications such as televisions, monitors of measuring apparatus, and information terminals. Compared to other, more recently developed types of flat display devices however, CRTs are relatively large and heavy. Therefore, many applications substitute CRTs for liquid crystal display (LCD) devices, operating according to an electro-optical effect, plasma display panel (PDP) devices, operating according to a gas discharge, and electroluminescence display (ELD) devices, operating according to an electro-luminous effect, and the like.
Due to their low power consumption and compact, lightweight construction, LCD devices are extensively researched and implemented in various applications such as monitors for desktop computers and laptop computers, and the like, in addition to large-sized display devices (i.e., display devices having a display size of 20 inches or more). Many LCD devices can display images by changing the transmittance characteristics of ambient light passing through an LCD panel. However, a number of LCD devices display images by changing transmittance characteristics of light emitted from a light source of a backlight unit. Depending on the location of the light source with respect to the LCD panel, backlight units can be generally classified as either direct-type or edge-type.
Edge-type backlight units generally include a lamp unit provided along a lateral side of a light-guide plate that is disposed below an LCD panel. The lamp unit includes a fluorescent lamp as the light source emitting light, a lamp holder arranged at ends of the fluorescent lamp for protecting the fluorescent lamp, and a reflective sheet for reflecting light emitted by the fluorescent lamp into the light-guide plate. The light-guide plate then uniformly transmits the emitted and reflected light into the LCD panel. Because edge-type backlight units have a long lifespan and thin profile, they are typically incorporated within relatively small-sized LCD devices (e.g., monitors for laptop, desktop computers, etc.). However, as large-sized LCD devices become more common, research and development of direct-type backlight units has increased.
Direct-type backlight units generally include a plurality of fluorescent lamps provided below a lower surface of a light-diffusion sheet that is, in turn, disposed below an LCD panel. Because the entire surface of the LCD panel is illuminated with light emitted from the plurality of fluorescent lamps without the use of a light-guide plate, direct-type backlight units can transmit light to LCD panels more efficiently than edge-type backlight units and are, therefore, typically incorporated within large-sized LCD devices (e.g., large monitors, televisions, etc.).
Large-sized LCD devices are often driven for long periods of time. Therefore, the direct-type backlight units incorporated therein are also driven for long periods of time. If, for some reason (e.g., due to technical problems with the lamp, expiration of operational life of the lamp, etc.), a particular lamp within a direct-type backlight unit is no longer capable of emitting light (i.e., the lamp is “dark”), images expressed at portions of the LCD panel which are aligned with the dark lamp are not as bright as images expressed at surrounding portions of the LCD panel. Accordingly, LCD devices incorporating direct-type backlight units must be capable of being easily disassembled and re-assembled, facilitating replacement of the dark lamp.
FIG. 1 illustrates a perspective view of a related art backlight unit. FIG. 2 illustrates a cross sectional view of the related art backlight unit shown in FIG. 1.
Referring to FIGS. 1 and 2, the related art backlight unit includes a plurality of fluorescent lamps 31 and first and second supporter assemblies. The first supporter assembly includes first lower and upper supporters 41a and 43a, respectively, and the second supporter assembly includes second lower and upper supporters 41b and 43b, respectively. The first supporter assembly further includes first and third conductive layers 47a and 47c, respectively, and the second supporter assembly further includes second and fourth conductive layers 47b and 47d, respectively.
All of the fluorescent lamps 31 are the same size and shape and each fluorescent lamp 31 is separated from an adjacent fluorescent lamp 31 by a predetermined distance. Moreover, each fluorescent lamp 31 includes first and second external electrodes 32a and 32b provided at opposing ends thereof.
The first lower supporter 41a is spaced apart from the second lower supporter 41b by a predetermined distance that corresponds to the length of the fluorescent lamps 31. Moreover, the first and second lower supporters 41a and 41b (herein collectively referred to as “lower supporters 41”) each include a first plurality of grooves 45 for receiving lower portions (i.e., lower cross-sectional halves) of opposing ends of the fluorescent lamps 31. The first and second upper supporters 43a and 43b (herein collectively referred to as “upper supporters 43”) are aligned with the first and second lower supporters 41a and 41b, respectively, and include a second plurality of grooves 45 for receiving upper portions (i.e., upper cross-section halves) of opposing ends of the fluorescent lamps 31. Further, the lower surfaces of the lower supporters 41 and the lower surfaces of the upper supporters 43 are wider than their respective upper surfaces. It should be noted that the interior side surfaces of the lower and upper supporters 41 and 43, respectively, (i.e., the side surfaces adjacent to the fluorescent lamps 31) are reflective.
The first and second conductive layers 47a and 47b, respectively, are formed along the longitudinal lengths of the first and second lower supporters 41a and 41b, respectively, as well as on a surface including the first plurality of grooves 45. Similarly, the third and fourth conductive layers 47c and 47d, respectively, are formed along the lengths of the first and second upper supporters 43a and 43b, respectively, as well as on a surface including the second plurality of grooves 45.
Accordingly, when the lower supporters 41 are joined to corresponding upper supporters 43, the interior side surfaces of the joined first lower and upper supporters 41a and 43a, respectively, and the joined second lower and upper supporters 41b and 43b, respectively, taper toward each other from the top of the upper supporter 43 to the bottom of the lower supporter 41. Moreover, when the lower supporters 41 are joined to corresponding upper supporters 43, the first and second pluralities of grooves 45 accommodate opposing ends of the fluorescent lamps 31 to positionally fix and support the fluorescent lamps 31 within the related art backlight unit while the first to fourth conductive layers 47a to 47d contact the external electrodes 32a and 32b within the joined first and second plurality of grooves 45 to transmit power to the fluorescent lamps 31.
Referring specifically to FIG. 1, the related art backlight unit further includes a diffusion plate 100a and diffusion sheets 100b and 100c disposed above the upper supporters 43 which, together, uniformly diffuse light generated by the fluorescent lamps 31.
As is understood from the discussion of FIGS. 1 and 2, the related art backlight unit can be characterized as having a luminous area (corresponding to a central portion of the area between the first lower and upper supporters 41a and 43a and the second lower and upper supporters 41b and 43b) and a non-luminous area (corresponding to the area between the luminous area and the first and second supporter assemblies in addition to portions of the top surface of the first and second supporter assemblies that are directly below the display area of the LCD panel), wherein the luminance at the non-luminous area is half the luminance at the luminous area. Accordingly, the average luminance of the entire backlight unit is determined, in large part, by the size of the non-luminous area and the reflective characteristics of the interior side surfaces of the first and second supporter assemblies. Moreover, images expressed at portions of the LCD panel that are aligned with the non-luminous area of the backlight unit are less bright than images expressed at portions of the LCD panel that are aligned with the luminous area of the backlight unit.
One method known to reduce the size of the non-luminous area involves uniformly reducing the width of the lower and upper supporters 41 and 43. However, and as shown in FIG. 2, uniformly reducing the width of the lower and upper supporters 41 and 43 undesirably exposes the first and second external electrodes 32a and 32b between the first lower and upper supporters 41a and 43a and the second lower and upper supporters 41b and 43b. During operation of the backlight unit shown in FIG. 2, the exposed portions of the first and second external electrodes 32a and 32b cast shadows onto portions of the diffusion plate 100a and diffusion sheets 100b and 100c that are next to the supporters and, therefore, minimize the benefits obtained by reducing the width of the lower and upper supporters 41 and 43.
Referring to FIG. 3, shadows cast by the exposed portions of the first and second external electrodes may be eliminated by reducing the length of the first and second external electrodes 32a and 32b to a predetermined length “a” that corresponds with a longitudinal width of the first and second plurality of grooves 45 within the lower and upper supporters 41 and 43 having the uniformly reduced width. However, as the length of the external electrodes 32a and 32b decreases, the tube voltage of the fluorescent lamp 31 undesirably increases and must be controlled to be below 900 Vrms. However, when the tube voltage is below 900 Vrms, the efficiency at which the fluorescent lamps 31 are driven to emit light is decreased.
Referring to FIG. 4, to solve the problems discussed above with respect to FIGS. 2 and 3, the width of the lower and upper supporters 41 and 43 can be uniformly increased, along with the longitudinal width of the first and second pluralities of grooves 45, allowing the first and second external electrodes 32a and 32b to have increased lengths which are completely received within the first and second pluralities of grooves 45. However, as the width of the lower and upper supporters 41 and 43 uniformly increases, the extended top surface portion of the upper supporters 43 is adjacent to the diffusion plate 100a and, therefore, cannot reflect light emitted by the lamps 31. Accordingly, the size of the aforementioned luminous area decreases and the size of the non-luminous area increases when the thickness of the lower and upper supporters 41 and 43 is uniformly increased.
Referring to FIG. 5, to solve the problems accompanied by the unreflective top surface portion of the upper supporters 43 shown in FIG. 4, the width of the lower and upper supporters 43 can be gradually increased from the top of the upper supporters 43 to the bottom of the lower supporters 41, in proportion to the distance from the top of the upper supporters 43. Accordingly, an inclination angle of the interior side surface of the lower and upper supporters 41 and 43 can be increased from some initial inclination angle, as illustrated in FIGS. 2 to 4, to an adjusted inclination angle θ1, between 30° and 50°, as measured from a normal line. As a result, the first and second external electrodes 32a and 32b having the increased length can be completely accommodated within the first and second pluralities of grooves 45 while the surface area of the upper supporters 43 that is adjacent to the diffusion plate 100a can be minimized. However, the reflective interior side surfaces of the lower and upper supporters cannot effectively reflect light emitted by the lamps 31 when the inclination angle θ1 is greater than 22°. Accordingly, the backlight unit shown in FIG. 5 effectively minimizes the benefit obtained by reducing surface area of the upper supporters 43 that is adjacent to the diffusion plate 100a while increasing the length of the first and second external electrodes 32a and 32b that are completely received within the first and second pluralities of grooves 45.